1. Field of the Invention
The present invention relates to coloured glasses, in particular (fluoro)phosphate glasses which are coloured blue for use as filter glasses, the use of such coloured glasses as filter glasses and a process for producing the glasses.
The glasses of the invention can be used as optical band-pass filters, i.e. as filters having a more or less narrow wavelength range of high transmission (“transmission range” or “passband”) surrounded by two “barrier ranges” having very low transmission. Such glasses are used as optical glass filters, for example as colour correction filters in video cameras and digital cameras. Further fields of application are filters for blocking the NIR radiation of LEDs, e.g. in displays etc. Apart from a high transparency in the range from 350 to 650 nm, a “steep edge” in the adjoining UV and a very low transmission above 700 nm are typical of the glasses of the invention. NIR-blocking filters are also used in aviation/navigation, so that a certain colour position trueness is necessary in the case of high blocking (e.g. colour position white or green). While the UV range should preferably be completely blocked, for example to avoid damage to sensitive electronic arrangements by the high-energy radiation, the intensity of the incident radiation in the range above 700 nm should be attenuated, so that, for example when used in cameras, the red cast in the photograph caused by the CCD (charge-coupled device) sensor is compensated. This requires, for example, transmission values in the NIR of from about 10−5 to about 10−20 or about 10−22 at a fixed filter thickness.
2. Description of Related Art
(Fluoro)phosphate glasses are known in principle from the prior art for use as filter glasses. However, these glasses have the disadvantage that they have poor weathering resistance and they are difficult to produce as a result of the often very high fluorine content because fluorine itself and the fluorides of many glass components are volatile under the conditions of conventional production processes. Many attempts have therefore been made to optimize the compositions of (fluoro)phosphate glasses with the objective of obtaining glasses which firstly have good stability and secondly can be obtained via economical production processes.
Furthermore, the need for very thin filters for which the glasses have to be more strongly coloured is increasing. However, this leads to problems in glass production since higher contents of colour-imparting components such as CuO have to be added for more strongly coloured glasses. However, at these higher amounts, these colour-imparting components not only act as colour-imparting components but, as glass constituents, also have an influence on other physical properties of the glass and the ability for the glass to be produced.
It is therefore an object of the present invention to provide coloured glasses which solve the problems of the prior art.